The present invention concerns a process for the preparation, by an electrochemical route, of a catalytic composition for the polymerization of olefins, a catalytic composition for this polymerization, and an application of the catalytic composition to the polymerization of olefins. More particularly, the present invention relates to the polymerization of ethylene and the copolymerization of ethylene with at least one .alpha.-olefin.
Means of obtaining catalysts containing at least one derivative of a transition metal, also known as "Ziegler-Natta catalysts," and their use for the polymerization of a .alpha.-olefins have been known for a long time. Those skilled in the art have sought ceaselessly to improve these catalysts to increase their catalytic activity and/or influence the characteristics of the polymers obtained.
Work has also been carried out with the object of using Ziegler-Natta catalysts under high temperature conditions to permit a shorter contact time between the olefin to be polymerized and the catalyst. This would result in an increase of the productivity of the installations. The catalyst must, therefore, have a high stability which is compatible with these drastic conditions of polymerization as well as a high activity and a high initial rate of polymerization.
Increasing the catalytic activity of Ziegler-Natta catalysts is a constant preoccupation which aims simultaneously, by the utilization of smaller quantities of catalyst, to reduce the manufacturing cost of the polymer and to obtain polymers in which the level of catalytic residues is as low as possible. The latter objective has primarily the purpose of avoiding purification of the polymers and of obtaining, without purification, polymers which have no tendency to degrade mechanically and/or thermally, during or after conversion into finished products.
It is known from another source that compounds of titanium of valency II are very slightly active in Ziegler-Natta catalysis. Many titanium compounds exist at the maximum valency (IV) and their reduction leads to mixtures of compounds of titanium (III) and of titanium (II) which become less active as the level of titanium (II) increases. One problem in the improvement of Ziegler-Natta catalysts is, therefore, to obtain, by reduction of compounds of titanium (IV), compounds of titanium (III) essentially exclusive of compounds of titanium (II).
The applicant has now found that it is possible to resolve this problem by means of a new process for the reduction of compounds of titanium (IV). The catalytic compositions obtained by this process have a very high activity regardless of the conditions of polymerization.
It is known to prepare components of Ziegler-Natta catalysts by an electrochemical route. For example, U.S. Pat. No. 3,787,383 describes an electrolytic process for preparing certain bis(metal halide)methanes, such as bis(dichloroaluminum)methane Cl.sub.2 ALCH.sub.2 AlCl.sub.2. By this process, an electrolyte of formula HOAlCl.sub.2 is electrolysed in the presence of a methylene dihalide, such as CH.sub.2 Cl.sub.2, or a gem-dihalide between an aluminum anode and a cathode of the same metal or of an inert material, such as platinum or carbon. A derivative of a transition metal is added to the resulting activator to obtain the catalytic system. This transition metal derivative does not undergo reduction.
It is also known from U.S. Pat. No. 3,546,083 to prepare simultaneously the two components (catalyst and activator) of the catalytic system by electrolysis of a medium containing a methylene dihalide and an electrolyte, such as HOAlCl.sub.2. The anode is constituted by a transition metal associated with aluminum and the cathode, which is not consumed during electrolysis, is a metal or graphite. The preparation of the transition metal compound, therefore, takes place by oxidation of the metallic anode. It is not specified in what valency state the transition metal is found in the compound formed.